The molecular formula of a substance is the chemical formula that gives the actual number of atoms of each element in the substance.
A molecular formula is the same as or a multiple of the empirical formula.
For example, the empirical of carbon dioxide is CO₂ and the molecular formula is also CO₂.
Whereas, the empirical formula of ethane is CH₃ while the molecular formula of ethane is C₂H₆.

Finding Molecular Formula

Example
Given that the empirical formula of benzene is CH and its relative molecular mass is 78. Find the molecular formula of benzene. [Relative Atomic Mass: Carbon: 12; Hydrogen: 1]

Answer:
Let’s say the molecular formula of benzene is C_nH_n.

The relative molecular mass of C_nH_n
_{= n(12) + n(1)}
_{= 13n}
_{13n = 78}
_{n = 78/13 = 6}
_{Therefore, the molecular formula of benzene}
₌C₆H₆

Example:

What is the mass of metal X that can combine with 14.4g of oxygen to form X oxide with molecular formula X₂O₃. (RAM: O = 16; X = 56 )

Answer:

Number of mole of oxygen
= 14.4/16
=0.9 mol

From the molecular formula, we learn that the ratio of element X to oxygen X:O = 2:3
Therefore, the number of mole of X =0.9× 2/3 =0.6 mol

Number of mole,
n = mass/Molar mass
0.6 = mass/56*
mass = 33.6g

The mass of element X = 33.6g

*Molar mass of a substance = Relative atomic mass of the substance

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